The writer's acquaintance with the remarkable fossils under consideration was begun by the examination, in 1840, of portions of teeth from the new red sandstone of Coton End quarry, Warwickshire. The external characters of these teeth corresponded with those (fig. 66) which had previously been discovered by Professor Jaeger in the German Keuper forma tion in Wirtemberg, and on which the genus Mastodonsaurus had been founded.
The results of a microscopic examination of the teeth of the Mastodonsaurus from the German Keuper, and of those from the new red sandstone of Warwickshire, proved that the teeth from both localities possessed in common a very remarkable and complicated structure (fig. 67), to the principle of which, —viz., the convergence of numerous inflected folds of the external layer of cement towards the pulp-cavity,—a very slight approach was made in the fang of the tooth of the Ichthyosaurus, whilst a closer approximation to the labyrinthic structure in question was made by the teeth of several species of ganoid fishes, and by those of Archegosaurus.
Thus, inasmuch as the extinct animals in question mani fested in the intimate structure of their teeth an affinity to fishes, it might be expected that, if they actually belonged to the class of reptiles, the rest of their structure would manifest the characters of the lowest order,—viz., the Batrachia, the existing members of which pass, though not by the dental cha racter alluded to, yet by so many other remarkable degrada tions of structure, towards fishes.
In the same formation in Wirtemberg from which the laby rinthic teeth of the so-called Mastodonsaurus had been derived, a fragment of the posterior portion of the skull has been obtained, showing the development of a separate condyle on each ex occipital bone ; whence Professor Jaeger, recognizing the identity of this structure with the batrachian character above mentioned, founded upon the fossil a new genus of Batrachia which he called "Salantandrades giganteus." Subsequent dis coveries, however, satisfied the Professor that the bi-condylous fragment of skull, representing the genus Salanzandroides, belonged to the same reptile as the teeth on which he had founded the genus Mastodonsaurus. The following fossils, from the new red sandstone of Warwickshire, gave additional proof of the batrachoid nature of the genus to which those Transverse section of a tooth of the Labrinthodon (magn.)
fossils belong, with the establishment of five distinct species, one of which is most probably identical with the Mastodon saurus salamandroides of Professor Jaeger. In reference to the generic denomination Mastodonsaurus, it unavoidably re calls the idea of the mammalian genus Mastodon, or else a warn milloid form of tooth, whereas all the teeth of the reptile so called are originally, and most of them are permanently, of a cuspidate and not of a mammilloid form ; secondly, because the second element of the word, saurus, indicates the genus to belong to the saurian or lacertian order of reptiles. For these reasons, the writer has proposed to designate the genus in question Labyrinthodon, in allusion to the peculiar and cha racteristic structure of the teeth (fig. 67).
The specimens from British localities are referable to five species—viz., 1. Labyrinthodon salamandroides ; 2. L. leptogna thus ; 3. L. pachygnath,us ; 4. L. ventricosus ; and 5. L. latus ; and we shall here briefly notice the characters exhi bited by the bones assignable to the second, third, and fifth species.
Labyrinthodon leptognathus.—The remains of this species consist of fragments of the upper and lower jaws, two vertebrae, and a sternum. They were found in the new red sandstone quarries at Coton End near Warwick.
A dorsal vertebra from Coton End presents further evidence of the batrachian nature of the Labyrinthodon. It has con cave articular cavities at the extremities of the body,—a con dition now known among existing reptiles only in the Geckos, and in the lower or perennibranchiate division of Batrachians. It is a common structure in extinct Saurians, but the depth of the vertebral articular cavities in the Labyrinthodon exceeds that in the amphiccelian Crocodiles and in most Plesiosaurs. The body of the vertebra is elongate and sub-compressed, with a smooth but not regularly curved lateral surface, terminating below in a slightly-produced, longitudinal, median ridge ; and it exhibits the same exceptional condition in the reptilian class as do the vertebrae of existing Batrachians, in having the superior arch or neurapophysis anchylosed with the centrum. From each side of the base of the neural arch a thick and strong transverse process extends obliquely outwards and upwards.